I'm pretty sure Coopers uses malt extract, not molasses-- they're one of the few breweries (maybe the only one?) that also makes malt extract kits.
Since you are using hopped malt extract with a short boil time, a larger boil volume is not really necessary. If you were conducting a longer boil (60 minutes or more) and adding your own hops, then it would make a difference in both reducing caramelization of sugars (darkening the wort and introducing some caramel flavors) and increasing utilization of the hops (more efficient bittering).
On Deck: Jamil's Vanilla Robust Porter
Fermenting: Orange Blossom Mead
Kegs: Element 56 Pale Ale, Ron's Belgian Blonde, Summer'n Saison, Furloughktoberfest '09, Grateful Pale Ale, Sam Adams Cream Stout Clone, EdWort's Apfelwein